Nano coatings show big potential for energy storage

Kevin Leonard holds a quarter-sized ultracapacitor built through his spin-off company SolRayo. Photo: Jim Beal.

Sitting in his office, Civil and Environmental Engineering Professor Marc Anderson picks up a small vial of clear liquid and shines a laser pointer through it. A deep green line bisects the vial, evidence of nanoparticles suspended in the liquid.

This liquid is Anderson’s platform technology. With it, he can dip or spray oxide-based nanoparticle coatings on virtually any surface for applications ranging from air and water purification to corrosion resistance. The technology has yielded both licensing agreements and startup companies.

One such company is SolRayo, which Anderson and PhD student Kevin Leonard founded in 2007 to develop ultracapacitors for renewable-energy storage. “If you could couple batteries with ultracapacitors, instead of having a two-year-life device, you could have a 10- to 20-year device,” says Leonard.

While conventional chemical batteries wear out after hundreds of charges, ultracapacitors use ions to store energy and can charge and discharge hundreds of thousands of times. “We actually have done 3 million discharge cycles on a device in our lab,” says Leonard.

Simply, an ultracapacitor consists of two separate electrodes immersed in a liquid electrolyte. Pairing ions from the electrolyte with the positive or negative electrode, the ultracapacitor stores energy at the electrode-electrolyte interface.

High-surface-area carbon is among the most effective—and expensive—electrode materials, in part, because it is heavily processed to make it extremely porous. As an alternative, Leonard, Anderson and colleagues at SolRayo can coat less-expensive carbon or the high-surface-area carbon with carefully sized nanoparticles. While the latter results in a more costly, high-performance device, SolRayo is focusing on the former option, which could result in larger, less-expensive ultracapacitors.

Using their technology, Leonard and his team have built a quarter-sized ultracapacitor that can hold about 2 farads of charge. Currently, they are working on a proof-of-concept prototype that, at about 18 cubic inches, will hold 3,000 farads.

A subsidiary of California-based Enable IPC Corp., SolRayo received a $250,000 grant in 2008 from the Wisconsin Energy Independence Fund to develop its technology.